The present invention relates generally to systems and methods used to form patterns on semiconductor substrates and, more particularly, to a system and method to correct for distortion caused by bulk heating in a substrate.
In the manufacture of integrated circuits, an electron beam (or E-beam) lithography process may be used to form circuit patterns on a semiconductor substrate. In particular, a focused electron beam is accurately positioned and readily scanned on the substrate to expose radiation-sensitive material (e.g., electron beam resist), thereby xe2x80x9cwritingxe2x80x9d a pattern or a mask into the material. This process of writing masks introduces heat into the substrate upon which the writing is done. Such heat can cause non-uniform thermal expansion of the substrate which, in turn, can cause errors in the placement of the pattern being written. That is, bulk (or global) heating of the photomaskxe2x80x94caused by electron beam energy deposition during patterningxe2x80x94results in thermal expansion of the mask substrate, thereby producing errors in pattern placement.
The present invention provides a system and method which corrects for the thermal expansion of a substrate during the writing process of electron beam lithography, thereby preventing errors that would otherwise be caused by the thermal expansion. with this system and method, the amount of heat introduced into a substrate is calculated from pattern writing data which may specify, among other things, the patterns to be written onto the substrate, the power level of the electron beam, and the exposure times for the electron beam. The pattern writing data is used to manipulate one or more pre-computed or pre-calculated distortion maps from a finite element model, which simulate or model the thermal responses and/or distortions of a substrate at the time of writing. In one embodiment, a linear superposition technique may be employed with the finite element models to predict the distortions. Using the finite element models, as manipulated with the pattern writing data, an equal offset in the placement of a written feature can be introduced for each expected distortion during electron beam lithography so that when the substrate cools down (i.e., is no longer distorted), the written feature appears at the correct position on the substrate.
According an embodiment of the present invention, an electron beam writing system includes an electron beam patterning machine operable to emit an electron beam to form a pattern on a substrate. A computer control system, coupled to the electron beam patterning machine, has a plurality of pre-computed distortion maps. Each distortion map describes expected distortions of the substrate caused by bulk substrate heating resulting from exposure to the electron beam. The computer control system controls the electron beam patterning machine using the distortion maps in order to adjust for the expected distortions.
According to another embodiment of the present invention, a method includes the following: receiving pattern writing data from an electron beam patterning machine operable to emit an electron beam to form a pattern on a substrate; determining expected distortions of the substrate caused by bulk heating resulting from exposure to the electron beam using the pattern writing data and a plurality of pre-computed distortion maps; and controlling the electron beam patterning machine to adjust for the expected distortions.
According to yet another embodiment of the present invention, a storage medium includes a reference table for use with an electron beam patterning machine operable to emit an electron beam to form a pattern on a substrate. The reference table has a plurality of pre-computed distortion maps. Each distortion map describes expected distortions of the substrate resulting from exposure to the electron beam.
According to still yet another embodiment of the present invention, a computer control system is provided for controlling an electron beam patterning machine operable to emit an electron beam to form a pattern on a substrate. The computer control system includes a memory which stores a number of pre-computed distortion maps, each distortion map describing expected distortions of the substrate resulting from exposure to the electron beam. A processor, coupled to the memory, receives pattern writing data from the electron beam patterning machine. The processor generates control signals using the pattern writing data and the distortion maps. These control signals control the electron beam patterning machine to adjust for the expected distortions.
A technical advantage of the present invention includes using a finite element model to predict the bulk heating distortions of the photomask during electron beam patterning based on linear superposition theory. From these predictions, adjustments can be made to compensate for the distortions, thereby preventing the occurrence of errors due to inappropriate placement.
Another technical advantage of the present invention includes providing one or more pre-computed distortion maps which describe the thermal and/or mechanical responses of a substrate to electron beam patterning. The distortion maps are used, in conjunction with pattern writing data, to determine the distortions expected in a patterning process so that adjustments can be made for the same. Because the distortion maps are pre-computed, the computational time required to calculate the thermal distortions is significantly reduced.
Yet another technical advantage includes a linear superposition technique which may be employed to correct for distortion caused by bulk heating in a substrate. Such linear superposition technique is not limited to any particular mask writing strategy or pattern density distribution, and thus, has widespread applicability.